Crystal modulator



June 27, 1961 RIEBMAN CRYSTAL. MoDULAToR Filed July l5, 1959 2,990,522 CRYSTAL MODULATOR Leon Richman, Huntingdon Valley, Pa., assgnor to American Electronic Laboratories, Incorporated, Philadelphia, Pa., a corporation of Pennsylvania Filed `luly 13, 1959, Ser. N0. 826,767 6 Claims. (Cl. 332-52) This invention relates to crystal modulators and particularly to modulators for ultrahigh frequency signals.

Heretofore, modulators for ultrahigh frequency signals have involved considerable ultrahigh frequency signal loss and generally considerable losses of power of the modulation input. Furthermore, the modulation index has been quite low, particularly in the case of modulating the highest frequencies.

It is the general object of the present invention to provide a crystal modulator involving an arrangement for tuning the capacitance of the crystal. By providing such a tuning arrangement and utilizing a series disposition of a varactor in series in the ultrahigh frequency line, together with the provision of a high input impedance at the point of modulation, the foregoing deficiencies of prior modulating circuits are minimized.

The attainment of the broad object of the invention as well as that of other objects particularly relating t details of construction and arrangement will become apparent from the following description, read in conjunction with the accompanying drawing, in which the figure shows diagrammatically and in section a modulating system provided in accordance with the invention.

The outer sheath of a coaxial transmission line is indicated at 2. The inner conductor of this line is interrupted for the insertion of the modulation arrangement, and the input portion of the line is indicated at 4 and the output portion at 6. Located between these portions is the varactor 8 which has its terminals inserted at 10 and 12 into openings in the line sections 4 and 6. An ultrahigh radio frequency source is indicated at 14. The invention is particularly useful when this source supplies signals in the range from 50 megacycles up to 12,00() megacycles, though it will be evident that the advantages of the arrangement are `gained when the input signals to be modulated are outside these limits. The source may be any conventional one and hence need not be further described. The input is delivered to the conductor 4 through the connection 16 which may be of any suitable type, for example, a coaxial transmission line continuing that which is diagrammed in the drawing.

To provide a direct current and bias return, the end of the conductor 4 which is illustrated is connected to the sheath 2 through a choke indicated at 18 which may take the form of a coil or an "alternative form such as hereafter described and shown as applied to the modulation input.

The modulation and bias source is indicated at and may take any suitable form to provide modulation at any desired frequency together with a direct bias to set the operating point of the varactor. The input from the modulation and bias source 20 may be provided through a coaxial cable arrangement, which may be conventional and need not be further described, the central conductor being continued at 24 to enter through the sheath 2 and connect to the conductor section 6. One aspect of the present invention is concerned with the provision of a high impedance at ultrahigh frequencies at the point of this connection, and to this end the conductor section 6 is provided with a hole 26 through the mouth of which the conductor 24 extends, the conductor being secured to the conductor section 6 at the bottom of the hole by soldering or otherwise. The arrangement thus provided teilt ECE gives rise to a low impedance path for the modulation signal and a direct conductive connection for the D.C. bias. At the same time, the input arrangement presents a high impedance to loss of the ultrahigh frequency and modulated signal through the connection 24'.

The arrangement just dwcribed may also be provided in place of the choke 18, in such case a conducting wire being similarly connected between lthe sheath 2 and into an opening in the conductor section 4.

The modulated signal may be delivered in any suitable fashion from the conductor section 6, as through a continuation of the coaxial arrangement, and is made as indicated at 28 to a suitable delivery device such as a horn 32 associated with a waveguide 30. Such a coupling and delivering arrangement is conventional and need not be further described.

The principal aspects of the invention have to do with the varactor 8 and its associated parts. A varactor provides, essentially, a capacitance which is varied by means of a voltage provided across the varactor terminals. Its equivalent circuit is that of a variable capacitor in series with a substantially linear spreading resistance, this series arrangement being shunted by a high barrier resistance. Due to the spreading resistance, the Q is relatively low, but nevertheless the variable capacitance may be effectively tuned by a parallel inductance, and this is provided by a suitable coil 34 connected at one end 36 to one of the terminals of the varactor, and coupled to the other terminal capacitively, this capacitive coupling being p-rovided to avoid short circuiting of the modulation and bias signals. For this ptupose, it has been found effective to connect the end of the coil 34 to a conductive element 38 embracing an insulating dielectric covering 40 extending about the varactor terminal. The conductive element 38 is desirably spread out as indicated to provide an optimum path for the radio frequency. It will be evident that a small capacitance is thus provided having a very low impedance at the signal frequency but providing a high impedance to the modulation signal and an insulator to the direct bias signal. As a result of the arrangement, the coil 34 is essentially in parallel with the series arrangement of the capacitance and its spreading resistance provided by the varactor.

The coil 34 may be conveniently inset into a groove 42 in the ceramic or other insulating housing of the Varactor so as not to project and adversely affect the configuration presented to the ultrahigh frequency. Generally, at frequencies of interest, the coil is quite small, typically, for example, one-sixteenth inch in diameter and of a length of the order of one-half inch, involving six to ten turns.

It will be evident from the foregoing that the association of the coil 34 with the varactor provides a resonant arrangement which may be tuned to a frequency differing slightly from that of the radio frequency source. In the absence of modulation, the source will then ordinarily provide a frequency to one side of the resonant peak of the varactor-coil circuit. As the modulating signal is applied, the resonant frequency of the varactor-coil arrangement is shifted with presentation of a substantial variation of impedance to the incoming radio frequency signal, effecting amplitude modulation so that the output signal is a modulated one. The return of the modulation and bias signals is through the choke arrangement provided at 18 in one of the forms described. By reason of this choking arrangement and by reason of the high impedance presented to ythe radio frequency and modulated signals at the point of modulation input, radio frequency loss is held very low. At the same time, there is no substantial impedance applied to the modulation signal so that highly effective modulation is secured.

While the primary tuning with respect to the high frequency involved is achieved by the choice of a coil 34 of proper inductance, fine tuning adjustment is secured by varying the bias tot charge the varactor capacitance of the LC combination. l

It will be evident that various, changes in details of what has been described may be made without departing from the invention as defined in the following claims.

What is claimed is:

1. An amplitude modulating system comprising a sheath, a coaxial assembly of an input conductor, a vol-tage-adjustable capacitor and an output conductor in series within said sheath, said capacitor being between said input and output conductors, means supplying high frequency signals to said input conductor, means applying a modulating signal across said capacitor between said conductors to modify its effective capacitance, and an inductance shunting said capacitor to provide a resonant circuit tuned to a frequency approximating but at one side of said supplied frequency to provide between said input and output conductors an impedance having substantial variations with said modulating signal.

2. An amplitude modulating system comprising a sheath, a coaxial assembly of an input conductor, a voltage-adjustable capacitor and an output conductor in series within said sheath, said capacitor being between said input and output conductors, means supplying high frequency signals to said input conductor, means applying a modulating signal across said capacitor between said conductors to modify its effective capacitance, and an inductance shunting said capacitor to provide a resonant circuit tuned to a frequency approximating but at one side of said supplied frequency to provide between said input and output conductors an impedance having substantial variations with said modulating signal, said modulating signal applying means including high frequency chokes blocking loss of high frequency signals from said assembly.

3. An amplitude modulating system comprising a sheath, a coaxial assembly of an input conductor, a voltage-adjustable capacitor and an output conductor in series Within said sheath, said capacitor being between said input and output conductors, means supplying high frequency signals to said input conductor, means applying a modulating signal across said capacitor between said conductors to modify its effective capacitance, and an inductance shunting said capacitor to provide a resonant circuit tuned to a frequency approximating but at one side of said supplied frequency to provide between said input and output conductors an impedance having substantial variations with said modulating signal, said modulating signal applying means including high frequency chokes blocking loss of high frequency signals from said assembly, at least one of said chokes being provided by a cavity in one of said conductors and a connection extending into said cavity and connected to the last mentioned conductor at the bottom of said cavity.

4. An amplitude modulating system comprising a sheath, a coaxial assembly of an input conductor, a voltage-adjustable capacitor and an output conductor in series within said sheath, said capacitor being between said input and output conductors, means supplying high frequency signals to said input conductor, means applying a modulating signal across said capacitor between said conductors to Vmodify its eective capacitance, an inductance shunting said capacitor to provide a resonant circuit tuned to a frequency approximating but at one side of said supplied frequency to provide between said input and output conductors an impedance having substantial variations with said modulating signal, and a small capacitance coupling one terminal of said capacittor to said inductance to block modulating currents from said inductance.

5. An amplitude modulating system comprising a sheath, a coaxial assembly of an input conductor, a voltage-adjustable capacitor and an output conductor in series within said sheath, said capacitor being between said input and output conductors, means supplying high frequency signals to said input conductor, means applying a modulating signal across said capacitor between said conductors to modify its eifeotive capacitance, an inductance shunting said capacitor to provide a resonant circuit tuned to a frequency approximating but at one side of said supplied frequency to provide between said input and output conductors an impedance having substantial variations with said modulating signal, and a small capacitance coupling one terminal of said capacitor to said inductance to block modulating currents from said inductance, said small capacitance being provided by a spreading conductor embracing an insulating sheet surrounding said terminal.

6. An amplitude modulating system comprising a sheath, a coaxial assembly of an input conductor, a voltage-adjustable capacitor and an output conductor in series within said sheath, said capacitor being between said input and output conductors, means supplying high frequency signals to said input conductor, means applying a modulating signal across said capacitor between said conductors to modify its effective capacitance, and an inductance shunting said capacitor to provide a resonant circuit Vtuned to a frequency approximating but at one side of said supplied frequency to provide between said input and output conductors an impedance having substantial variations with said modulating signal, said inductance being inset into a groove in the body of said capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,182,377 Guanella Dec. 5, 1939 2,461,307 Antalek Feb. 8, 1949 2,783,378 Vogeley et al. Feb. 26, 1957 FOREIGN PATENTS 362,377 Great Britain Nov. 27, 1931 

